Abstract: A tube joint made of metal such as zinc and the like having a plurality of protruding longitudinal knurling tools. Each of the knurling tools has a wide root end and a narrow pointed end at the central part of the upper and the lower surfaces and of the right and the left sides of a square prism which has each corner cut off to form a fixed groove from the pointed end to the root end. A slight arcuate-shaped protuberance is provided at the central part of each of the longitudinal knurling tools. The knurling tool with its protuberance forms a joint rod having an outer diameter equal to, or slightly larger than the inner diameter of a square pipe into which the outer diameter of the diagonal in the protuberant part of the knurling tool corresponding to the square prism is inserted, in arbitary directions at the connecting part thereof. A plurality of joint rods are connected at their root ends to form the tube joint.